The world-wide production of chlorine, about 45 million tons per year, is carried out in electrolytic cells of different types; among these, the diaphragm electrolytic cell, by means of which about 22 million tons of chlorine per year are produced, has a great relevance.
A diaphragm electrolytic cell is generally composed of four main parts, as known to the experts in the art: a copper anodic base, lined with a protective titanium sheet, an anodic package, consisting in a multiplicity of anodes disposed in parallel rows and secured to said base, a carbon steel cathodic body, comprising a plurality of cathodes upon which a porous diaphragm is deposited, secured to a current distributor and disposed in parallel rows so that they can be intercalated to the above anodes according to a so-called “finger-type” geometry, and a cover, usually of chlorine-resistant plastic material provided with the nozzles for feeding the brine and discharging the product chlorine.
In consideration of the high number of installed cells (about 25000 world-wide), of the high amount of energy involved in their operation (about 60 millions of MWh/year) and of the continuous increase in the cost of electricity, the cell diaphragm technology has been, in the course of the years, remarkably improved. Among the many technological innovations which offered the major contributions for decreasing the energy consumption, the following must be noticed:                the replacement of the traditional graphite anodes with box-shaped perforated metallic anodes (the so-called “box” type anodes) made of titanium, coated with electrocatalytic material based on noble metals and/or oxides thereof.        the replacement of fixed sized “box” anodes with the so-called “expandable anodes”, as disclosed in U.S. Pat. No. 3,674,676, allowing for the reduction of the interelectrodic gap.        the suppression of the above interelectrodic gap through the introduction, within the expandable anodes, of means for exerting a pressure between the anodes and the diaphragm, as disclosed in U.S. Pat. No. 5,534,122        the evolution of the expandable anode through the introduction of the double expander, as disclosed in U.S. Pat. No. 5,993,620, whereto a lower ohmic drop is associated.        
It may be observed that the cited innovations are all directed to improve the performances in terms of energetic consumption, by means of either an increase of the electrocatalytic activity, or an optimisation of the electrode structure, or again through the reduction of the interpolar gap and the increase in the mass transfer (lower bubble effect and higher electrolyte circulation) obtained through small modifications which do not imply a substantial redesign of the cell structure and thus of easy implementation and reduced costs.
Other solutions proposed in the past provide a modification of the cell, and in particular of the cathodic package, directed to increase the electrodic surface thereby decreasing the current density at a given applied total current, and as a consequence the cell voltage and the overall energetic consumption.
A further issue of present great relevance is given by the need of increasing the electric load and thus the production; such need is often in contradiction with the lack of a suitable area allowing the installation of additional electrolytic cells. In the co-pending unpublished International Application PCT/EP 02/10848, a solution allowing the increase of the cell active surface with the same projected area is disclosed, by means of the construction of a cell made of a plurality of vertically overlaid modules provided with the conventional interdigitated anodes. This solution is in itself promising, although entailing quite substantial investment costs.
It is an object of the present invention to provide a new diaphragm electrolytic cell overcoming the drawbacks of the prior art.
In particular, it is an object of the present invention to provide a diaphragm electrolytic cell comprising a multiplicity of overlaid modules of anodes and cathodes, the anodes of at least part of the modules allowing for a substantial reduction of the construction cost.